Lithium secondary batteries have been widely used as power sources of portable devices after they have emerged as small, lightweight, and high-capacity batteries since 1991. Recently, in line with the rapid development of electronics, communications, and computer industries, camcorders, mobile phones, and notebook PCs have appeared and undergone continuous and remarkable development. Accordingly, the demand for lithium secondary batteries as a power source for driving these portable electronic information and communication devices has increased day by day.
Lithium secondary batteries have limitations in that their lifetime rapidly decreases as charge and discharge are repeated. In particular, the above limitations are more severe at high temperature. The reason for this is due to a phenomenon that occurs when an electrolyte is decomposed or an active material is degraded due to moisture in the battery or other effects, and the internal resistance of the battery increases.
Cathode active materials for a lithium secondary battery, which have been actively researched and developed, may include LiNiO2, LiMn2O4, LiFePO4, Li(NixCoyMnz)O2. However, with respect to LiNiO2, the synthesis thereof may not only be difficult but there may also be limitations in thermal stability, and thus, commercialization is difficult. With respect to LiMn2O4, some have been commercialized as low-cost products, but life characteristics were poor due to structural distortion (Jahn-Teller distortion) caused by Mn+3. Also, since LiFePO4 is inexpensive and has excellent stability, a significant amount of research has currently been conducted for the application of LiFePO4 for a hybrid electric vehicle (HEV). However, the application to other areas may be difficult due to low conductivity.
Thus, Li(NixCoyMnz)O2 is a material which is currently very much in the spot light as a cathode active material alternative to LiCoO2. This material is less expensive than LiCoO2 and may be used in high voltage and high capacity applications. However, Li(NixCoyMnz)O2 has limitations in that rate capability and life characteristics at high temperature may be poor. In order to address the above limitations, a significant amount of research, for example, has been conducted by a method of coating the surface of a cathode active material with a metal having good conductivity or a method of doping the cathode active material with a material such as aluminum (Al), magnesium (Mg), titanium (Ti), zirconium (Zr), tin (Sn), calcium (Ca), silver (Ag), and zinc (Zn). With respect to the coating, a wet method has been used, but actually it has a great limitation of increasing the cost in mass production. Currently, reports about the improvement of the properties of the above metal through dry doping tend to increase.
For example, Korean Patent No. 10-277796 discloses a technique of coating a cathode active material with a metal oxide by coating the surface of the cathode active material with a metal, such as Mg, Al, cobalt (Co), potassium (K), sodium (Na), or Ca, and heat treating the cathode active material in an oxidizing atmosphere.
However, limitations, such as the occurrence of cracks in the cathode active material and a reduction in capacity or output of a secondary battery, are difficult to be addressed. Therefore, a cathode active material, which may reduce side reactions between electrolyte and active material during charge and discharge, may minimize the reduction in the capacity or output of the secondary battery, and may improve life characteristics, is required.